a. Field of the Invention
The present invention relates generally to the construction of ropes, and, more particularly, to a braided construction which is particularly suited to large diameter ropes made of artificial fiber materials.
b. Background
Conventional braided construction is widely used in the manufacture of ropes for various uses (it will be understood that there are also types of rope construction which are not braided, such as plain laid or cable laid ropes). In conventional braiding, twisted yarns are woven repeatedly under and over each other, so that each yarn follows a generally helical path over the full length of the rope. The angle of the path depends on the tightness of the braid, commonly expressed in terms of a number of "picks" per unit length.
The individual yarns are twisted prior to braiding primarily to form the fibers into a coherent bundle. This also serves to increase the translational efficiency of the yarns slightly (as used herein, the term "translation efficiency" expresses the relationship between the breaking strength of the yarn and the combined breaking strength of the fibers which form the yarn, in terms of a percentage of the latter value), by helping to ensure that the individual fibers are more evenly loaded. However, while a small amount of twist (e.g., 1/2 turn per inch for a 3/8" diameter bundle) will produce a slight increase in translational efficiency (perhaps 10%, for example), twisting the yarn further causes a rapid decline in tensile strength. This is because with further twisting the fibers on the outside of the bundle begin to follow significantly longer paths than those toward the inside, so that in use the shorter fibers may be overloaded before they can elongate sufficiently for the longer fibers to begin taking a strain; this is a particular problem when working with low-elongation fiber materials, some of which are able to stretch only about 2-4% before breaking.
Although the industry has for many years made braided ropes using twisted yarns, this technique has exhibited several serious drawbacks in connection with certain recent advances. For example, there is an increasing need for very large diameter braided ropes (e.g., for use on large escort tugboats, in single point mooring systems, in the offshore oil industry, etc.), but because of existing equipment and other reasons most braided rope is limited to using a certain number of strands (e.g., 12-strand braided rope, 8-strand braided rope, etc.). Consequently, when making larger diameter ropes the conventional practice has been to simply increase the diameter of the yarns.
The approach of simply "scaling up" the system has not proven successful, however, especially when using comparatively new, low-elongation fiber materials, such as polyester, Kevlar.TM., and UHMWPE (ultra-high molecular weight polyethylene) materials such as Spectra.TM., for example. This is in part because the formation of large diameter yarns requires multiple stage twisting when working with synthetic fiber materials (because of their rough surface texture natural fibers tend to form a firm bundle upon initial twisting, but synthetic fibers are much smoother and slippier). As was noted above, the degree of twist which is necessary to achieve optimal translational efficiency is typically very small, regardless of the fiber material, and over-twisting is very detrimental to tensile strength. When performing multiple-stage twisting, however, it is virtually impossible to give the yarn a satisfactory level of coherency without exceeding the optimal amount of twist, with the result that translational efficiency suffers severely. On the other hand, while it is possible to use a very loose twist so as to avoid a loss of translational efficiency, this results is an unacceptably low level of coherency and produces a loose, "sleazy" yarn which is prone to snag damage and is otherwise unsuitable for commercial service. In short, when using large-diameter twisted yarns, it is difficult or impossible to achieve an acceptable degree of coherency along with a high level of translation efficiency.
Processes exist by which twisted UHMWPE and other yarns can be successfully stretched at elevated temperatures to achieve a high degree of translational efficiency without damaging the individual fibers. Even these processes have a practical upper limit in terms of the diameter of yarn which can be successfully treated in a production operation, however, and for the present this limit is well below the diameter of the yarns which are required for the construction of very large braided ropes using conventional manufacturing techniques.
Another problem stems from the need to splice the yarns multiple times when braiding long pieces of rope. To illustrate this, reference is made to FIG. 5, which shows a conventional braider machine 01 having a plurality of bobbins 02 mounted on a table 03 for developing an intertwining rotation (note: since the braider machine does not itself constitute a part of the present invention and is well known to those skilled in the relevant art, only an overview of the mechanism will be provided here). As the bobbins move about, the yarns are woven over and under one another and drawn upwardly through a collar 05 by a take-up reel 07. Then, as each bobbin runs out of yarn, it is necessary to stop the machine and splice in the yarn from a fresh bobbin. It is not generally practical to splice the ends of twisted yarns together (since they tend to simply unravel into an incoherent mass), and so the conventional practice has been to place the fresh bobbin in an open area 08 at the center of the table and then lead the end of the yarn upwardly into the core of the rope, as indicated by arrow 09. The machine is run to form another 20-30 feet (typically) of rope, and then the yarn from the old bobbin is cut off and the new bobbin is mounted in its place at the edge of the table.
This technique is conventionally referred to as a "braider interchange," and although used for many years it is unsatisfactory in many respects. Firstly, because this is a frictional splice it will always represent a weak spot in the rope. Also, the 20-30 ft overlap represents a costly wastage of material, especially when using expensive fibers. Still further, this type of splice becomes extremely difficult to perform when braiding large diameter ropes. This is because the spools which are needed to carry the larger-diameter twisted yarns are much bigger and more tightly packed on the table of the braider machine than is shown in FIG. 5, with the result that there is simply no space in the middle of the table in which to position the replacement spool (scaling up the size of the machines to provide more room is too expensive to be a practical option). Moreover, when the twisted yarns are very large it becomes difficult to handle the heavy, stiff end of the yarn and feed it up into the core of the rope.
The use of large twisted yarns to form the strands of the rope also makes it very difficult to make repairs to conventional braided ropes when individual strands become damaged in service. For example, a single yarn may become snagged, cut, or otherwise damaged while the remainder of the rope remains intact. The inability to repair the individual yarns, however, means that an entire length of the rope must be discarded, at great cost.
Accordingly, there exists a need for a method of constructing large diameter braided ropes wherein a high degree of translational efficiency is achieved, especially when using low-elongation fiber materials. Furthermore, there exists a need for such a method of construction which allows large-diameter braided rope to be manufactured without having to use excessively large twisted yarns. Still further, there exists a need for such a method of construction which permits faster, more efficient splices to be formed between the ends of individual strands during the construction of the rope. Still further, there is a need for such a method for constructing braided ropes which permits individual strands to be spliced so as to repair damage without having to discard an entire length of the rope.